Wednesday 29 August 2012 18.54 EDT
First published on Wednesday 29 August 2012 18.54 EDT

Heavy rain, high winds and floodwaters swept over Louisiana and Mississippi on Wednesday, as Isaac was downgraded from a hurricane to a tropical storm and a fortified levee system appeared to have saved New Orleans from disaster.

Rescuers picked up dozens of residents who had ignored warnings to leave low-lying areas, seven years to the day after hurricane Katrina devastated the city.

Power lines were cut and debris littered the streets, prompting authorities in New Orleans to declare a dusk-to-dawn curfew. Louisiana officials said they would intentionally breach a levee in Placquemines parish, south of New Orleans, as Isaac lumbered inland from the Gulf of Mexico.

Authorities feared many residents would need help after a night of torrential rain and harsh winds knocked out power to more than 700,000 households and businesses.

Isaac has top sustained winds of 70mph (112kph), just below the hurricane threshold of 74mph (119kph).

Army Corps spokeswoman Rachel Rodi said the city's bigger, stronger levees were withstanding the assault. "The system is performing as intended, as we expected," she said. "We don't see any issues with the hurricane system at this point."

New Orleans mayor Mitch Landrieu issued a curfew for the city. Police cars had been patrolling the nearly empty streets since Isaac began bringing fierce winds and heavy rains to the city Tuesday night.

Rescuers in boats and trucks plucked a handful of people who became stranded by floodwaters in thinly populated areas of south-east Louisiana. Emergency officials in Plaquemines Parish said floodwaters had flowed over an 8ft levee between the Braithwaite and White Ditch districts.

Convoys of people clutching children, pets and bags streamed in driving rain down state highway 23 while a fleet of ambulances with blaring sirens headed in the opposite direction towards the Mississippi to aid people still in danger.

A YMCA centre in Belle Chase, just south of New Orleans, became an improvised refugee station for 125 people after two other nearby shelters filled up.

"The waters had reached the seventh step of my porch and the lawmen told us we had to evacuate the premises," said Veraldine Garrison, 72, who arrived with four adult children.

"All we had was the wind and the rain. The wind and the rain." Like others, she feared for her home. "There ain't nothing you can do with mother nature. You can't control that water."

Beside her, huddled in a blanket, sat a woman rescued from her rooftop, too exhausted or traumatized to speak.

Some evacuees complained they were forced to leave pets and valuables behind. "My chihuahuas, Tiki and Tutu, are still in my trailer home," said Thomas Wyman, 80, slumped in a plastic chair. "And my money, my medicine, it's all there."

Jim Gabour, who lives in the Faubourg Marigny neighborhood of New Orleans, just next to the French Quarter, said that with electricity knocked out, trees downed and the wind still up, residents were in survivalist mode – and did not expect relief until the weekend.

"No electricity, no running water and 80mph gusts," Gabour wrote in an email on Wednesday. "Right now, cutting trees off house in high winds and stinging rains. Cell batteries gone soon. Land lines not working."

He added later: "Generator too erratic for computer. No electricity, cable or land lines until weekend. Will have to turn off generator soon, and be in dark again, as we have to ration gas and no stations open. And it is suffocatingly hot."